Foreigner and re-entry into China proper

I'm a westerner, currently working and living in Beijing on an M-type Chinese visa. I'd like to visit Tibet before my visa expires, but my work visa allows for zero re-entries into China. If I visited there on one of your tours, would I have to re-apply for another Chinese visa, or is TBT covered as part of China, as far as that visa is concerned, and I could re-enter once the tour ends?